Agree with what everyone else has to say about what will happen here - you are headed for heaps of duplicates. Firstly, I'd say that this scenario is too complex for a post on the community. You need a Marketo consultant (partner/professional services) to advise you ASAP and build a strategy that fits your circumstances. having done this hundreds of times, when I read your post I had a million questions that would go into advising you what to do. 1. When you say that you are doing a massive dump into Salesforce, how is this data coming in? If you are using the Salesforce API rather than list import functions you get a heck of a lot more flexibility in terms of deduping that data based on email or other criteria on your way in. In your scenario of syncing the Marketo data as leads back into Salesforce, you could do a dedupe on SFDC after that, but it would depend on what format your data is in, and whether the data coming from Marketo as leads causes you an issue. 2. Is the email activity you've done in Marketo so far worth saving? You could push all your email reports out into another database so you have the results, if you can use the API you could push out activities into a database first, empty your Marketo and start again. 3. I'd want to know how many fields we are talking about in Marketo that you've already uploaded data into - just the standard fields or have you built custom fields in Marketo that now need to be remapped into your Salesforce sync? (there are more considerations here than just data). 4. Have you looked at third-party tools to help you de-dupe Marketo if you did go down that road? Sometimes that solution is just easier if the data volumes are not that big. Do yourself a favour here, hire a consultant to help you plan your next steps, or you could spend weeks/months undoing a decision based on something somebody told you on a message board that doesn't have a hope of considering ALL the factors in your environment.
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